1. Field of the Invention
The present invention relates to a buttonhole closing device for a lockstitch bar tacking sewing machine.
2. Description and Problems of the Related Art
A lockstitch bar tacking sewing machine performs a cycle of sewing operations to form a given number of stitches for bar tacking sewing, button sewing, etc., and bar tacking seams are formed using the lockstitch bar tacking sewing machine. There is known in the art an electronic bar tacking sewing machine which is electronically controlled in feeding sewn products in X (longitudinal) and Y (lateral) directions to form a seam pattern thereon. The bar tacking sewing machine which is controlled by way of a feed cam in feeding the sewn products to form a seam pattern thereon. The rotation of the arm shaft is controlled by way of a motor without the intervention of a clutch mechanism therebetween, and the like.
In conventional buttonholing, overedge chain stitches are formed around a buttonhole and thereafter bar tacking is performed at the start and end portions of the overedge chain stitches using the sewing machines of this kind. At the time of performing such a bar tacking operation, the buttonhole is closed before the bar tacking is performed thereon. This straightens loose buttonholes which are formed in the center of the overstitch chain stitches for receiving a button therein so as to put the start and end portions of the overedge chain stitches together along a line. There are known two ways of closing the buttonhole, i.e., that of moving a first movable clamp foot on the left and a second movable clamp foot on the right toward the center line of the buttonhole interlocking with the vertical movement of a clamp foot unit including the first and second movable clamp feet, and that of directly driving the first and second movable clamp feet toward the center line of the buttonhole by pneumatic actuators etc. separately.
They have, however, the following technical problems. That is, when the first and second movable clamp feet are moved toward the center line of the buttonhole interlocking with the vertical movement of the clamp foot unit for closing the buttonhole, the clamp foot unit is lowering while the first and second movable clamp feet thereof move toward the center line of the buttonhole, so that the buttonhole is liable to be closed incorrectly due to the difference in thickness of the sewn products and consequently the buttonhole is closed incompletely and a uniform seam cannot be formed. When the first and second movable clamp feet are directly driven toward the center line of the buttonhole separately by driving units each composed of a pneumatic actuator etc. for closing the buttonhole, the driving units arranged adjacent to the first and second movable clamp feet narrow a free space for handling the sewn products to set the same in place, which deteriorates operability and lowers productivity.